Frameshift mutagenesis will be studied in the yeast Saccharomyces cerevisiae using a combined genetic and biochemical approach. Evidence from many sources suggests that frameshifts in procaryotes occur in monotonous runs of bases (GGGG or AAAA). Our evidence suggests that yeast frameshift suppressors suppress the four base code word GGGG. We will sequence the mutant glycine tRNA in our suppressor strains to obtain direct evidence for the molecular change in the frameshifts. In addition this sequence work should provide important evidence on the effect of the non-Mendelian determinant (PSI ion) on glycine tRNA. The glycine frameshift suppressors will be mapped by conventional and aneuploid mapping. Suppressors of a second class of ICR revertible mutations (which could be CCCC) will be analyzed biochemically and genetically. Mutations which enhance the frequency of these frameshift mutations will be characterized. Some of these may affect repair systems and others the permeability of mutagens. Two new classes of potential frameshifts in yeast will be isolated and characterized. We will search for frameshifts among spontaneous polar his4 mutants. Suppressors of such mutants will be isolated and the tRNAs characterized by double label experiments. Experiments are designed to uncover ICR-170 revertible mitochondrial (MIT-) mutations in an attempt to characterize the susceptibility of the mitochondrial genetic system to frameshift mutagenesis. Ultimately, these studies could lead to the construction of a strain capable of detecting frameshift mutations induced by carcinogens in both the nuclear and the mitochondrial genomes.